In recent years, usage environment for oil well tubes and boiler tubes, etc., has become increasingly hostile. This has led to a higher level of requisite properties of seamless tubes for use in these tubes. For example, higher strength and more excellent corrosion resistance are required in oil well tubes for use in oil wells that tend to be deeper and more corrosive. Tubes for use in nuclear power generation facilities and chemical plants are required to be excellent in corrosion resistance, particularly in stress corrosion cracking resistance in the environment where these tubes are exposed to high-temperature water including high-temperature pure water and chlorine ions (Cl−). In order to satisfy these requirements, more of seamless tubes made of high Cr-high Ni alloy (also referred to as simply “high alloy”, hereinafter) containing large amounts of Cr, Ni, and Mo have been used as oil well tubes and the like.
High alloy seamless tubes may be produced with Mannesmann tube-making process such as a Mannesmann-mandrel mill process, a Mannesmann plug mill process, and a Mannesmann assel mill process. These tube making processes include the following steps:
(1) piercing-rolling a round billet heated at a predetermined temperature into a hollow blank (hollow shell) through a piercing mill (piercer);
(2) elongation-rolling the hollow blank through an elongation-rolling mill (e.g. mandrel mill, plug mill); and
(3) diameter-adjusting-rolling the elongation-rolled blank through a diameter-adjusting-rolling mill (e.g. sizer, stretch reducer) into a finished tube having a predetermined outer diameter and wall thickness.
Round billets for use in the manufacture of high alloy seamless tubes are produced by casting molten alloy whose chemical composition is appropriately adjusted in a melting process into cast slab with a rectangular cross section in a continuous casting process, and rolling the continuously cast slab to the round bar with a desired diameter by using grooved rolls in a blooming and billet-making process.
A high Cr-high Ni alloy has deformation resistance approximately 2.4 times as high as that of carbon steel, and approximately twice as high as that of 13% Cr steel or BBS steel, for example, and thus processing-incurred heat is significantly generated, resulted from shearing deformation due to hot working. A high alloy round billet is subjected to larger shearing deformation at its both ends than that at its central portion during piercing-rolling the high alloy round billet. Hence, during piercing-rolling, while the both ends of the high alloy billet are subjected to larger shearing deformation, and at the same time, significant processing-incurred heat is generated there, resulting in great increase in temperature of the billet. Consequently, such a high alloy hollow blank produced through the piercing-rolling is likely to have grain boundary melting cracking (referred to as “tube end cracking”, hereinafter) in a circumferential direction at the ends of the tube.
The tube end cracking also extends in a tube axis direction within the wall of the hollow blank, and the cracking remaining in the wall is further elongated in a tube axis direction in the subsequent elongation-rolling process and diameter-adjusting-rolling process, which results in product defective. In a hollow blank having the tube end cracking, the end of the hollow blank where the cracking exists needs to be cut off as a defective portion. As a result, defective portions to be removed from products are increased, which decreases a product yield, resulting in deterioration of the production cost.
It has been ardently desired to prevent tube end cracking from being generated during piercing-rolling in the production of a seamless tube of high Cr-high Ni alloy. Increase in temperature at the billet end portions due to processing-incurred heat during piercing-rolling is one of causes to generate the tube end cracking; thus, to satisfy the desire, such a solution can be considered that piercing-rolls the billet at a temperature lowered in advance, thereby suppressing melting in crystal grain boundaries at the end portions of the billet. Decrease in heating temperature of the billet, however, may arouse such a problem that deformation resistance of the billet be increased, which may increase load onto the piercing mill, and cause troubles in the operation. Hence, the solution to decrease the heating temperature of the billet is not practical in the case of rolling a high alloy billet.
The prior art pertinent to these facts is as follows.
Patent Literature 1 discloses a technique of focusing on outer surface flaws generated during a billet-making process where a continuously cast slab is subjected to a blooming and rolling process to yield a round billet, in production of a seamless tube for a bearing made of high-carbon chromium steel containing C of 0.7 to 1.5 mass % and Cr of 0.9 to 2.0 mass %, and employing a solution to prevent occurrence of such outer surface flaws, thereby producing a seamless tube excellent in surface quality. The technique disclosed in this Patent Literature is directed to rolling of high-carbon chromium steel, and performs blooming and billet-making under a condition that specifies a relation among a long side length W (mm) and a short side length H (mm) of a cross section of a cast slab, and a diameter D (mm) of a round billet.
Patent Literature 2 discloses a technique of focusing on inner surface flaws of a seamless tube caused by δ-ferrite produced in a central segregation of a continuously cast slab, in production of a seamless tube of 13% Cr steel (martensite-based stainless steel), and employing a solution to prevent occurrence of the inner surface flaws. The technique disclosed this Patent Literature is directed to rolling of 13% Cr steel, and specifies a chemical composition of this steel, specifies a heating temperature of a billet during piercing-rolling, and also specifies a flatness ratio (long side length/short side length of cross section) of the cast slab to be 1.8 or more.